The invention relates to a process for manufacturing metal sheet gaskets coated with elastomer, as are often used as cylinder head gaskets or as compressor gaskets in refrigerant circuits.
To date, the coating of such metal sheet gaskets has been carried out by the metal sheetings being transported from roll to roll and having a continuous coating applied to them and by the corresponding gasket sheets then being punched out of the endless material and further processed to produce the finished gaskets.
The problem with this process is that the cut edges resulting from the punching remain uncoated and, therefore,
a) are susceptible to corrosion, and
b) offer at the cut edge possible points of attack for liquids and gases which may cause the elastomeric coating on the surface of the metal sheets to become detached.
This problem was resolved in a different way of manufacturing metal sheet gaskets coated with elastomer wherein the metal sheets for the gaskets were first punched out of the endless material, and the punched-out sheets were coated with an elastomeric substance while passing through a curtain of liquid in a so-called flow coating process. The disadvantage of this way of manufacturing the metal sheet gaskets is that adjacent to edges of the gaskets, in particular, at openings such as, for example, combustion chamber edges of cylinder head gaskets or other passages for liquid in the gasket, and also at beads and shoulders, coating irregularities, for example, in the form of so-called runs or sags, occur on the underside of the sheets. The irregularities easily attain approximately the thickness of the desired coating and more, and so these negatively affect the function of the gasket. Furthermore, with this process, especially if the problem of the coating irregularities is to be kept within reasonable limits, only a relatively thin coating can be applied in one run, and so the coating has to be carried out several times if thicker coatings are desired. This involves not only a delay in the manufacture or manufacturing installations of correspondingly larger dimensions, but also additional operations, for example, if the accumulations of coatings were to cause such disturbances that they had to be eliminated by a separate punching operation. All of this negatively affects the cost factor in the manufacture of such known gaskets to a considerable extent.
A factor which is also not to be overlooked is the intermediate drying of the coating substances which is often necessary when several coats are applied.
The object of the present invention is to provide a process for the manufacture of metal sheet gaskets coated with elastomer wherein the coating with elastomer can be carried out with greater cost efficiency and avoidance of the above-mentioned disadvantages.
This object is accomplished in accordance with the invention in the process described at the outset by the elastomeric coating being applied to the individual metal sheet in a spray coating process.
Surprisingly, it has been found that the coating substances, although they have a distinctly higher viscosity than is usually the case in the spraying of paints, result in a problem-free way in a coating of the metal sheets of a metal sheet gasket with elastomer, in particular, using conventional spraying equipment, which is of excellent quality. Furthermore, with the spray coating technique, excessive accumulations of coatings on the underside of the sheets, in particular, at cut edges, beads or shoulders of the metal sheet layers, are avoided, and so additional machining such as, for example, subsequent punching to remove accumulations of coatings is dispensed with.
Furthermore, with the spray coating process a considerably larger layer thickness can be applied in one run than with the flow coating process used so far. In particular, an elastomeric coating with a layer thickness of distinctly more than 16 xcexcm is possible with the inventive spray coating process. With this, there are no coating irregularities to impair the function of the gasket, and also a slight loading of the metal sheets with elastomer on the underside thereof due to spray mist has no disadvantageous consequences.
An elastomer coating operation is preferably carried out with a coating substance having a viscosity at approximately 23xc2x0 C. of 10 to 120 seconds, preferably 30 to 90 seconds, efflux time with a 4 mm German Industrial Standard cup. On the one hand, these viscosities still allow good working with the spray coating technique, and, on the other hand, ensure that considerable layer thicknesses on the metal sheet, if desired, can be achieved in one run.
The equivalents in cps to the viscosity values given in seconds for the 4 mm standard up are:
Preferred elastomeric coatings which are manufactured in accordance with the inventive process have a layer thickness of 3 to 60 xcexcm. It is to be noted that all these layer thicknesses can be applied in a single operation, i.e., in a single run, optionally with two directly successive coating procedures, by the spray coating apparatus.
The elastomer solutions used as coating substance can have a solids content of 5 to 40% by weight, with carbon black admixtures, fillers, auxiliary vulcanizing agents., etc. being understood as solids. It is important that the coating substance used for the elastomeric coating should include an elastomer solvent or solvent mixture so the elastomer is present in the substance in a dissolved form and not as a solid.
For easy handling of the spray coating process, in particular as regards the technical side of the installation, it is desirable for the solvent or solvent mixture to be recoverable. The solvent of the coating substances or the solvent mixture is preferably selected from elastomer-dissolving alcohols, aromatic compounds, esters and/or ketones having a boiling point in the range of 100 to 200xc2x0 C. These solvents or solvent mixtures meet the requirements stated hereinabove.
Further preferred are solvents or solvent mixtures which include esters with boiling points in the range of 100 to 180xc2x0 C., preferred up to approximately 150xc2x0 C., and/or contain ketones with a boiling point in the range of 100 to 160xc2x0 C., preferred up to approximately 120xc2x0 C.
The inventive process can be carried out in a particularly economical way if the coating substance is recovered by the transport means with which the metal sheets are conveyed through the spray coating installation. This will often be a conveyor belt, and, in particular, one will recover the portions of the coating substance which pass through the combustion chamber openings in the metal sheets to the belt. This is possible in a particularly simple way by the pot life of the coating substance being adapted so as to be more than one hour.
In this way, only slight amounts of coating substance are lost in the coating of the metal sheets, and the process attains a particularly high cost efficiency.
The auxiliary vulcanizing agents referred to hereinabove are adapted to the overall system of the coating substance and are selected such that they are only activatable by thermal action or by high-energy rays so that a crosslinking of the cross-linkable elastomer systems is only achieved at the dry stage and so that pot lives of the elastomer solution of one hour and more are achievable.
The inventive process thus makes it possible to produce the necessary openings, beads, shoulders, etc. before the coating operation for with the spray coating it is ensured that no unacceptable accumulations of coatings will occur in these problem areas during the elastomer coating operation. This, furthermore, has the great advantage that the punched edges of the metal sheets are also coated with elastomer and, therefore, do not offer any points of attack for streams of lubricant, streams of coolant, etc., which can result in the elastomeric coating becoming detached from the metal sheet.
A coating substance which comprises per 100 parts by weight elastomer up to 150 parts by weight carbon black, up to 150 parts by weight fillers, up to 20 parts by weight crosslinkers, up to 50 parts by weight resins and 250 to 9500 parts by weight solvent or solvent mixture, and, optionally, up to 20 parts by weight auxiliary agents in the form of antioxidants, flow improvers, nonstick agents and accelerator additives is preferably used for the elastomeric coating.
The carbon black components are preferably added in the range of 20 to 80 parts by weight, and furnace blacks and thermal carbon blacks are preferably used, of these, in particular MT, FEF and HAF blacks, with the abbreviation MT standing for medium thermal, FEF for fast extruding furnace and HAF for high abrasion furnace.
As fillers, in particular calcium carbonate, calcium silicate, calcium metasilicate, mica, silicic acid, heavy spar, titanium dioxide, magnesium oxide, lead oxide, talc, kaolin, quartz and/or chlorite are suitable, with preferred filler amounts being 20 to 80 parts by weight.
As crosslinkers/auxiliary vulcanizing agents, 3 to 50 parts by weight are preferably used, and preferred crosslinkers/auxiliary vulcanizing agents are sulfur plus accelerator, crosslinker resins, aminic crosslinkers, bisphenolic crosslinkers, peroxides, triallyl isocyanurate, etc.
The resins referred to in the basic formula are preferably used within a range of 5 to 15 parts by weight and can comprise phenolic resins and epoxy resins.
The elastomer referred to several times hereinabove is preferably selected from NBR, FPM, AEM, ACM and/or EPDM, with, in particular, NBR and FPM being preferred, and with the abbreviation NBR standing for nitrile rubber, FPM for fluorinated rubber, AEM for ethylene-acrylate rubber, ACM for acrylate rubber and EPDM ethylene-propylene-diene rubber.
In order to optimize the recovery of coating substance, the completely punched and completely shaped metal sheets are transported on a conveyor belt with a closed surface through a coating station in which the elastomeric coating is applied in the spray coating process, and the portions of the coating substance which pass onto the conveyor belt surface, for example, through openings in the metal sheets, are removed from this and used again.